Copy a mesh along an oblique direction and scale
In edit mode in one of the objects already there, select one of the already existing edges parallel to that direction, then go to the transformation orientation panel and click the + button.  This should create a custom 'Edge' transformation orientation, which you can use to arrange things as needed, in new or different objects, or with part of objects in edit mode.

To make a thinner rail panel, you could duplicate a panel, go into edit mode, select one of the vertical edges and then transform it along the Y direction of this custom Edge transform orientation - though in this case you can also do it possibly even quicker by using the Edge Slide tool (default hotkey G, then G again, when an edge is selected).  Scaling will not work here as the object is not symmetrical along that axis, and the opposite corners will distort, losing you the parallel lines.